University of Florida psychologist Gregory Webster, a co-author of the study, told NPR that teams with darker-colored jerseys were penalized for two minutes more per game than teams with white jerseys. Those with black jerseys were given an average of 1,528 penalty minutes a season while those wearing white were assessed 1,386.

The researchers studied more than 52,000 NHL games over three decades, and they were helped by a rule change in 2003, when home teams switched from wearing white jerseys at home to their darker uniforms. That helped eliminate any bias referees might have had for home teams.

The study doesn’t answer why teams with black jerseys were more frequently penalized, but Webster has several theories. First, those wearing dark jerseys are more easily seen on ice. But that doesn’t account for the fact that there was no visible spike in penalties for players wearing colors other than black. Another theory is that darker colors somehow make players more aggressive. And a third is that there’s simply a bias toward white and against black.

“There is this very strong cultural association that comes through in how we think about colors in terms of being associated with good and black with bad,” Webster told NPR. “Many of us are raised from childhood with some of these associations. And over time, we develop a kind of cognitive bias. That’s been shown time and time again in social psychology.”